It is known to form an engine air cleaner as a bank of straight flow tubes, each tube having swirl means at its mouth to cause dust to be centrifugally thrown outwardly toward the tube wall. Dust near the tube wall is drawn from the main stream by a "scavenger" fan; cleaned air is taken from the core zone of the tube through a small diameter take-off tube extending into the downstream end of the flow tube.
The centrifugal separator action is relatively ineffective on particles smaller than 5 microns; therefore it has been proposed to add electrostatic separator action to enhance overall collection efficiency. In one arrangement an ionizer wire negatively charged to approximately 15-20 KV is extended through the tube on the tube centerline; the tube wall is at ground potential to establish a radiating flow of particle-charging negative ions. The resultant negative charges on the particles and the radial electrical field from the ionizer wire to the tube accelerate or enhance outward migration tendencies, especially of the smaller particles, thereby improving overall collection efficiency.
The electrostatic separator action causes some particles to precipitate and adhere rather strongly to the tube side wall. Therefore it was necessary to periodically rap or vibrate the tube in the radial and/or axial direction in order to dislodge the particles sufficiently to permit the scavenger air to carry them away. Under some circumstances the particles were jarred with such force as to be re-entrained into the clean air stream; at other times the collected particles resisted the jarring forces to prevent fluidization into the scavenger stream. Even when properly applied, rapping or vibrating requires special shock mounting of the tube; mechanical wear on the mounts is a problem. Therefore, the use of rappers as a dislodging expedient is not entirely satisfactory.
The present invention provides means for removing collected particles from the tube surface without the necessity for rapping or vibrating the tube. Instead the "removing force" comprises a dielectric layer on the inner surface of the tube. Experiments indicate that such a layer tends to weaken the attractive forces between the grounded metal surface and the collected particles sufficiently to enable the areodynamic forces to have the desired scavenging action without the need for rapping.